1. Field of the Invention
Exemplary embodiments generally relate to an electric charging apparatus and an image forming apparatus using the electric charging apparatus, such as printers, copying machines, facsimiles, etc. Further, exemplary embodiments also relate to discharging electrons and reducing deterioration in an electron discharging member.
2. Discussion of the Background
Background electronic photograph processes use corona discharge to uniformly electrify a photoconductor. A corona discharge device usually includes a platinum or tungsten wire electrode with a diameter of about 50-200 μm or a needlelike stainless steel electrode provided in a conductive case. A high-voltage bias of direct current or alternate current is applied between the electrode and the case to ionize molecules of air near the electrode. A photoconductor near the electrode can be evenly discharged using the ions, but ozone and nitrogen oxides are generated because of ionizing air. It is known that the amount of generated ozone and nitrogen oxides becomes as much as 4-10 ppm after a 60-minute electrification. If ozone remains in an image forming apparatus at a high concentration, the surface of the photoconductor can be oxidized, thereby lowering the light sensitivity and/or electrification ability of the photoconductor, and reducing image forming quality. Further, ozone in the image forming apparatus can also accelerate deterioration of the other parts used in the image forming apparatus.
Nitrogen oxides react with the moisture in air generating nitric acid, and react with metal etc., generating a metal nitrate. Although these reaction products have a high resistance in a dry environment, under highly damp conditions, they react with moisture in the air and have a low resistance. Therefore, if a thin film of nitric acid or a nitrate is formed on the photoconductor surface, an unusual image such as flowing images can be formed. This is because the nitric acid and nitrates generated have a low resistance due to absorbing moisture from the air, and thereby a potential of electrostatic latent image on the surface of the photoconductor is decreased. Since the generated nitrogen oxides remain in the air without being decomposed after electric discharge, adhesion of the compounds generated from nitrogen oxides on the photoconductor surface can occur during a non-discharge period or a non-operation period of image forming processes. The compounds can permeate the inside of the photoconductor as time passes, thereby causing deterioration of the photoconductor. A cleaning method is known in which the adhesion layer on the surface of the photoconductor is removed by shaving off the photoconductor surface little by little. However, this cleaning method is costly and the deterioration problem can remain.
In corona electrification methods, the applied voltage can be as high as about 4-10 kV to cause the electric discharge between separated electrodes. The electrification potential can change depending on the electrification time according to a rotation speed of the photoconductor and its passing by the electrification component. In order to obtain the required electrification potential (400V-1000V), it is necessary to enlarge the width of the case electrode in the direction of the rotation of the photoconductor, especially when the photoconductor rotation speed is high. Therefore, there is a problem that it is hard to miniaturize an image forming apparatus with a quick print speed. In recent years, proximity roller electrification methods have become widely used. In these electrification methods, a direct current (dc) or alternate current (ac) bias is applied between an electrification component (charge roller) supported so as to be close to a photoconductor and the photoconductor, thereby causing electric discharge therebetween, so that the photoconductor is electrified. The electrification phenomenon in accordance with Paschen's electric discharge rule is used in this proximity roller electrification method. The desired electrification potential is obtained by making a large potential difference therebetween which is the same as an electric discharge starting potential. In the case of applying an ac bias, the direction of electric field alternatively changes with time between the proximity electrification component and the photoconductor to thereby repeat electric discharge and reverse electric discharge. Although there is an advantage that the electric field is uniformly equalized by electric discharge and reverse electric discharge using the ac bias, the risk of photoconductor contamination due to electric discharge is still very high.
Thus, electrification of the photoconductor using Paschen electric discharge still includes the risk that electric discharge generation products can adhere to the photoconductor surface or that the photoconductor surface becomes oxidized by an active gas produced by the electric discharge. Therefore, the surface of the photoconductor still must be minutely shaved off in order to maintain image quality. However, it is desirable to avoid having to shave off the photoconductor surface to extend the life of the photoconductor. This loss of life is the trade-off from the use of shaving to prevent degradation of image quality. A contacting charging apparatus in which the electrification component contacts the photoconductor to electrify the photoconductor has also been proposed and used. For example, a roller-like electrification component contacts the photoconductor and is rotated with the photoconductor to charge the photoconductor. This contacting charging method only produces a small amount of ozone. For example, the amount of generated ozone after a 60-minute contacting electrification using a dc voltage bias is about 0.01 ppm. This value is smaller than that of the corona electrification method. Further, since the applied voltage is low, it has advantages of reducing the cost of the power supply and reducing the difficulty of designing the electric insulation. Of course, the problems caused by the above-mentioned ozone and NOx can also be reduced.
In the above-mentioned contacting charging method, a narrow space is formed at the position of the contact or near the proximity portion, the electric discharge in accordance with Paschen's law is made, and the photoconductor is charged. Further, a method of applying a dc voltage that is higher than the electrification starting potential to a conductive component, or promoting equalization of electrification by applying an oscillating voltage superimposed with an ac voltage on the dc voltage equivalent to target electrification potential can be used. However, if an ac voltage is applied, the direction of electric field alternatively changes between the electrification component and the photoconductor. Electric discharge and reverse electric discharge are repeated as noted above. Although there is an advantage that the electric field is uniformly equalized by electric discharge and reverse electric discharge, the amount of generated ozone and nitrogen oxides increases due to increased current, for example. Depending on this current increase, ozone of no less than 3 ppm can be generated after 60-minute electrification similarly to the corona electrification method. As another method, contacting the above-mentioned conductive component with the photoconductor and charging the trap level on the photoconductor surface can be performed. In this method, the conductive component (charge roller) is generally used to conveniently control the shape or condition of the contacting portion.
However, contacting the roller conductive component to the photoconductor includes many disadvantages. For example, when a copy machine is stopped for a long period of time, the roller in contact with the photoconductor can deform because the electrification component is usually a rubber material. Moreover, since rubber is a material which easily absorbs water, its resistance can largely change according to an environmental water content change. Furthermore, rubber needs several kinds of plasticizers and an active agent for providing elasticity without deterioration. In order to distribute conductive pigments, it is common to use an auxiliary distributing agent. That is, since the surface of the photoconductor is made of an amorphic resin, such as polycarbonates or acrylics, the surface has low resistance to the effects of the above-mentioned plasticizers, active agents, and the auxiliary distributing agents. Moreover, a foreign substance can be present between the electrification component and the photoconductor when using the contact electrification method, so that the electrification component is polluted as a result, and poor electrification can occur. Furthermore, since the roller is in contact with the photoconductor, the photoconductor becomes polluted after a long period of time, therefore a poor image, such as one with abnormal horizontal lines, can be generated.
Recently, attention has been directed to a method using electronic discharge material relative to electrification technology. Research on carbon nano material has rapidly progressed in recent years and suggests a high electron discharge ability. It has been reported that a carbon nanotube tip portion can be made to have durability by specifying the constituent factor of the carbon nanotube, and it can be used to be in or out of contact with a charging apparatus. It is also known that an image bearer is electrified as a source of electronic discharge based on the Paschen electric discharge between the parallel plates and the specified field intensity between the charging apparatus surface and the charged body. However, since carbon nano material is organic matter, the carbon nano material itself can be oxidized with the oxygen atom which is excited by an emitted electron just like in the atmosphere of an electronic photograph system. Thus, the carbon nano material can be decomposed by combustion, so that there is a problem that a desired life expectancy cannot be attained due to a resulting very weak structure.